Although applicable to any desired stiffening elements, the present invention and the problems on which it is based will be explained in more detail with reference to frames for stiffening a fuselage shell of an aircraft.
Fuselage shells for aircraft are normally produced using a so-called lightweight structure from an outer skin, which is reinforced on the inside by a two-dimensional structure composed of stringers, which run in the longitudinal direction of the aircraft, and frames, which run transversely with respect to the longitudinal direction of the aircraft, as stiffening elements.
In this case, aluminum and aluminum alloys have been used for decades as the conventional material for the outer skin, stringers and frames. However, these are increasingly being replaced by fiber composite materials, in particular by carbon-fiber-reinforced plastic (CFRP) since they make it possible to reduce the total weight of the aircraft, and therefore to use less energy during flight, with an aircraft fuselage with the same stiffness and strength. Further advantages of fiber composite materials over aluminum materials are little material fatigue and the absence of corrosion.
However, the processing of fiber composite materials requires techniques which often differ fundamentally from the techniques used for processing aluminum materials. By way of example, aluminum frames for fuselage sections of different diameter can be produced by bending an extruded profile to the respective desired curvature, after which it is milled to the precise intended shape. However, a large number of lamination apparatuses, in each case of appropriately different forms, must be provided to produce correspondingly differently shaped frames composed of carbon-fiber-reinforced plastic, in which apparatuses fiber mats can be draped and can be laminated using epoxy resin in the respectively desired intended shape. In addition to increased investments costs, this involves considerable logistic effort, resulting in high production costs.
In order to allow the total weight of the aircraft to be reduced by the large-scale use of carbon-fiber-reinforced plastic, while at the same time restricting the production costs, it is therefore worthwhile to use frames composed of an aluminum material in conjunction with an outer skin and/or stringers composed of carbon-fiber-reinforced plastic. This results in the problem of there being an electrochemical potential between the aluminum and the carbon-fiber-reinforced plastic, leading to corrosion on the aluminum side if the two materials make contact.